International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 18.3, pp. 390-391
Section 18.3.2.5. Torsion angles
a
Pharmaceutical Research, Roche Diagnostics GmbH, Max Planck Institut für Biochemie, 82152 Martinsried, Germany, and bMax-Planck-Institut für Biochemie, 82152 Martinsried, Germany |
Since torsion angles are generally more adequately determined by protein structures of typical resolutions, there is less need to derive parameters from the CSD for refinement purposes. Further, distributions derived from small molecules may not be representative of torsion angles among proteins, since typical fragments lack `typical' protein secondary structure. This kind of environmental dependence will affect softer parameters such as dihedral angles more than bonds or angles. However, since the torsion-angle distribution is a function not only of potential interactions of the peripheral groups but of the electronic character of the bond itself, some questions may be best examined by the study of chemical fragments. One such feature might be the distributions for aromatic residues. Statistical distributions of side-chain orientations show the apparent stability of an eclipsed conformation, particularly for ( conformer, Fig. 18.3.2.3). If the relative dearth of secondary structure leads to atypical torsion-angle distributions in proteins, it must be conversely expected that torsion-angle statistics derived from the set of all proteins will be less applicable, for example, to helical proteins than statistics derived from mostly helical proteins. This illustrates the dilemma of selecting the ideal statistical database for protein refinement.